The present invention relates generally to semiconductor lasers, and specifically to tuning of semiconductor laser systems.
The increase in demand for greater bandwidths in communications is driving interest in semiconductor laser systems. In order to accommodate the high bandwidths, a typical system may use 80 or more wavelength division multiplexed (WDM) channels, each channel being optically time division multiplexed (OTDM) at rates of 40 Gbit/s or more. Such systems are required to generate narrow pulses, having widths of the order of several picoseconds. Semiconductor laser chips can generate coherent radiation at wavelengths of the order of 1.5 xcexcm (approximately 200 THz), and so can form an integral part of such a system. However, a drawback common to all monolithic semiconductor lasers is that control of their operating wavelengths, repetition rates, and pulse widths, is not sufficiently accurate for the WDM/OTDM system described above.
FIG. 1 is a schematic diagram of a semiconductor laser system 10, known in the art, which overcomes some of the drawbacks described above. A system similar to that of FIG. 1 is described in U.S. Pat. No. 5,305,336 to Adar et al. which is incorporated herein by reference. System 10 comprises a single-section semiconductor laser device 12 having a substantially 100% reflecting facet 14, and an antireflection-coated facet 16. Radiation from facet 16 is coupled into a fiber optic 18, which has a Bragg grating 20 inscribed in the optic. In some embodiments known in the art, grating 20 comprises a multi spectral features fiber Bragg grating (MSFFBG). Grating 20 acts as a second partial reflector, causing device 12 and section 22 of the fiber optic to function as a fiber grating laser (FGL) that generates coherent radiation at a wavelength defined by the grating.
In the system described by Adar et al, 20 ps pulses at repetition rates of 2.5 GHz were produced by actively mode-locking the cavity, forming a mode-locked FGL (ML-FGL). The linear chirp of the grating allowed tuning of the repetition rate to a desired frequency. However, this was also accompanied by self-tuning of the emission wavelength of the laser over the width of the grating. Furthermore, the length of the pulses produced, and use of the single-section laser device which was modulated as a whole, limit the repetition rate.
It is an object of some aspects of the present invention to provide apparatus and a method for producing high repetition rate optical pulses.
In a preferred embodiment of the present invention, a laser is implemented by optically coupling a monolithic device having an active semiconductor lasing region with a multi spectral features fiber Bragg grating (MSFFBG) inscribed in a fiber optic. The laser is able to support a plurality of longitudinal modes of vibration. The device comprises a semiconductor wavelength tunable filter (WTF) which acts as a relatively wide band-pass filter, enabling the laser to be tuned to a number of adjacent modes to the virtual exclusion of the others. Preferably, the device also comprises a saturable absorber (SA) which is modulated with a radio-frequency signal and which is situated in an operating section of the device so that the laser is mode-locked to generate short pulses. Combining the active lasing region, the saturable absorber, and the WTF in the monolithic device, and optically coupling the device to the MSFFBG, forms an efficient compact lasing system that is tunable and that is able to generate short optical pulses at a specific wavelength with a high repetition rate.
In some preferred embodiments of the present invention, the monolithic device also comprises a phase-change region and a passive waveguide region. Addition of these two regions to the operating section of the device enables the SA region to be accurately positioned, in a two step process, at an optical center of a cavity defined by the device and the MSFFBG. In a first step the SA region is physically implemented at an approximate optical center. In a second step the phase-change region is tuned to adjust a phase delay within the cavity so that the SA region is accurately at the optical center.
The WTF may be implemented either as a transmission filter or as a reflection filter. If implemented as a transmission filter, the WTF is preferably formed as a grating assisted co-directional coupler, which may be tuned using current injection and/or by changing the temperature of the WTF. As a transmission filter, the WTF may be positioned substantially anywhere within the operating section of the monolithic device.
If the WTF is implemented as a reflection filter, it is most preferably positioned adjacent to an end facet of the device, acting there as a highly reflecting mirror. The reflection WTF is preferably implemented as a distributed Bragg reflector (DBR), which may be tuned using current injection and/or by changing the temperature of the DBR. Alternatively, the reflection WTF is implemented as a multi spectral features Bragg grating (MSFBG), which may be tuned by methods known in the art.
There is therefore provided, according to a preferred embodiment of the present invention, a laser, including:
a grating structure, including two or more gratings generating a first plurality of different wavelength peaks for reflection of optical radiation therefrom; and
a semiconductor device, including a gain region which is operative to amplify the optical radiation, and a wavelength tunable filter (WTF) region which is adapted to filter the optical radiation, the device being optically coupled to the grating structure so as to define a laser cavity having a second plurality of cavity modes, which are selected by tuning a wavelength pass-band of the WTF region to overlap with one of the wavelength peaks of the grating structure.
Preferably, the semiconductor device includes a saturable absorber which is adapted to be modulated so as to pulse the optical radiation.
Further preferably, the semiconductor device includes a highly reflective coated facet and an anti-reflection coated facet which bound the device, and the saturable absorber is positioned adjacent one of the facets.
Preferably, the semiconductor device includes an active phase-change region and a passive waveguide region which are adapted to position the saturable absorber centrally within an optical length of the laser cavity.
Further preferably, the active phase-change region implements a phase delay within the laser cavity so as to locate the saturable absorber at an optical center of the laser cavity.
Preferably, the WTF is implemented as a transmission band-pass filter.
Preferably, the semiconductor device includes an anti-reflection coated facet, and the WTF is implemented as a reflection band-pass filter located adjacent the anti-reflection coated facet.
Preferably, the grating structure includes a multi spectral features fiber Bragg grating (MSFFBG) inscribed in a fiber optic.
Further preferably, a width of a spectral feature of the MSFFBG is adjusted so as to determine a number of the plurality of the cavity modes.
There is further provided, according to a preferred embodiment of the present invention, a method for generating a laser output, including:
providing a grating structure generating a first plurality of different wavelength peaks for reflection of optical radiation therefrom;
optically coupling a semiconductor device to the structure so as to define a laser cavity, the device comprising a gain region which is operative to amplify the optical radiation and a wavelength tunable filter (WTF) region which is adapted to filter the optical radiation; and
tuning a wavelength pass-band of the WTF region to overlap with one of the wavelength peaks of the grating structure so as to generate a laser output in a second plurality of cavity modes defined by the overlap.
Preferably, the semiconductor device includes a saturable absorber (SA), and including modulating the SA so as to pulse the optical radiation.
Further preferably, the semiconductor device includes a highly reflective coated facet and an anti-reflection coated facet which bound the device, and including positioning the saturable absorber adjacent one of the facets.
Preferably, the method includes locating an active phase-change region and a passive waveguide region within the semiconductor device so as to position the saturable absorber centrally within an optical length of the laser cavity.
Further preferably, the method includes utilizing the active phase-change region to implement a phase delay within the laser cavity so as to locate the saturable absorber at an optical center of the laser cavity.
Preferably, the WTF is implemented as a transmission band-pass filter.
Preferably, the semiconductor device includes an anti-reflection coated facet, and the WTF is implemented as a reflection band-pass filter located adjacent the anti-reflection coated facet.
Preferably, the grating structure includes a multi spectral features fiber Bragg grating (MSFFBG) inscribed in a fiber optic.
Further preferably, the method includes adjusting a width of a spectral feature of the MSFFBG so as to determine a number of the second plurality of the cavity modes.
Preferably, optically coupling the semiconductor device to the grating structure includes butting the device to the structure.
Preferably, optically coupling the semiconductor device to the grating structure includes positioning a lens intermediate the device and the structure.
Further preferably, the grating structure includes a multi spectral features fiber Bragg grating (MSFFBG) inscribed in a fiber optic, and the lens is integral to an end of the fiber optic.
Preferably, tuning the resonant wavelength includes varying a temperature of the WTF region.
Alternatively, tuning the resonant wavelength includes varying a current injected into the WTF region.
Preferably, the grating structure is implemented to determine a number of the second plurality of the cavity modes, so as to control a pulse width of the optical radiation.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: